Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...

The reason I used Snake as a code name in Metal Gear is that a snake was the most appropriate symbol of a living being that hides his presence, and sneaks without making any noise. The reason why I didn’t use any specific snake names like cobra, anaconda or viper was because the protagonist is the player.

Like for of all endings of the any series, the appearance of strongest enemy was a must in Metal Gear Solid. It’s the Snake who can surpass the Snake. Thus I brought about a ‘clone.' Solid vs. Liquid. That was Metal Gear Solid.

As I developed a sequel, a third Snake was needed. Since both Solid and Liquid express an actual natural state. Obviously the next would be 'Gas' but Gas Snake would be like a human made of gas, and that’s not a nice name. So I borrowed from the physics terms ‘solidus/liquidus’. Solidus is not a real state but indicates the border between liquid and solid.

And there you have it. Mystery solved. I guess it makes sense... sort of.